The present invention relates to the field of disc drive data storage devices. More particularly, this invention relates to fly height control for a read/write head over patterned media.
An important device in any computer system is a data storage device. Computer systems have many different places where data can be stored. One place for storing massive amounts of data and instructions is a disc drive. The disc drive has one or more discs, each with two surfaces on which data is stored. The surfaces are coated with a ferro-magnetic medium with regions that are magnetized in alternate directions to store the data and instructions. The coated surfaces are computer-readable media holding computer-readable data and computer-readable and computer-executable instructions. The discs are mounted on a hub of a spindle motor for rotation at an approximately constant high speed during the operation of the disc drive. An actuator assembly in the disc drive moves magnetic transducers, also called read/write heads, to various locations relative to the discs while the discs are rotating, and electrical circuitry is used to write data to and read data from the media through the read/write heads. Data and instructions are stored in the media of one or both of the surfaces of each disc. The disc drive also includes circuitry for encoding data and instructions written to the media and for decoding data and instructions read from the media. A microprocessor controls most operations of the disc drive, such as transmitting information including instructions or data read from the media back to a requesting computer and receiving data or information from the requesting computer for writing to the media.
Information representative of data or instructions is stored in tracks in the media. In some disc drives, information is stored in a multiplicity of concentric circular tracks in the media on each disc. In other disc drives, information is stored in a single track that forms a continuous spiral in the media on each disc. A read/write head is positioned over a track to write information to or read information from the track. Once the operation is complete, the read/write head may be controlled to move to a new, target track, to write information to or read information from the target track. The movement takes place in the following modes. The read/write head is moved along an arc across the media of a disc in a seek mode to position it near the target track. The read/write head is then positioned over the target track during a track-and-follow mode, also called a tracking mode, to read or write the information stored in the target track. Servo information is read from the target track by the read/write head, and a feedback control system determines a position error signal from the servo information. If the read/write head is not in a correct position, it is moved to a desired position over the target track in response to the position error signal.
Each read/write head is typically located on a slider that is supported by the actuator assembly. The actuator assembly is controlled to position the read/write head over the media of one of the discs. Each slider is attached to a load spring supported by an arm. The arms in the actuator assembly are rotatably mounted to an actuator shaft through bearings and are rotated about the actuator shaft by a voice coil motor to move the read/write heads over the media. The bearings and the actuator shaft are also called a pivot. The voice coil motor includes a voice coil mounted to the actuator assembly opposite to the arms. The voice coil is immersed in a magnetic field of an array of permanent magnets placed adjacent to the actuator assembly. The feedback control system applies current to the voice coil in a first direction to generate an electromagnetic field that interacts with the magnetic field of the magnets. The interaction of the magnetic fields applies a torque to the voice coil to rotate the actuator assembly about the pivot, and the actuator assembly is accelerated to move the read/write head to a new position. The feedback control system may then apply current to the voice coil in a direction opposite to the first direction to apply an opposite torque on the actuator assembly. The opposite torque may be used to decelerate the actuator assembly and position the read/write head over a target track. The opposite torque may also be used to accelerate the actuator assembly to a different position.
Each slider is a small ceramic block that flies over the media of one of the discs. When the disc rotates, air flow is induced between the slider and the media, causing air pressure which lifts the slider away from the media. The slider has an air bearing surface that is aerodynamically shaped to give the slider lift when air flows between the slider and the media. The load spring, described above, produces a force on the slider directed toward the media. The forces on the slider equilibrate such that the slider flies over the media at a nominal fly height. The fly height, also called clearance, is a distance between the slider and the media, and is a measure of an amount of air available to interact with the air bearing surface of the slider as it is aerodynamically supported over the media. The fly height of the slider affects the fly height of the read/write head carried by the slider, which is a distance between the media and the read/write head. The fly height of the read/write head should be approximately uniform so that the read/write head is capable of reading data from, and writing data to, the media.
Several variables affect the fly height of a slider. For example, fly height is impacted by a curvature of a disc, vibrations of the disc caused by the spindle motor, and roughness and defects in the media. Fly height is also affected by a variation in the aerodynamics of the slider due to changes in its orientation and position during flight.
The media may be patterned, and this also affects the fly height of a slider and the fly height of a read/write head. In conventional discs servo information is written as signals or bursts in servo wedges across the media. The bursts are used to determine a position of a read/write head relative to a track in the media. The bursts occupy a substantial amount of the surface of the media, and require a substantial amount of time and equipment to be written into the media. The bursts are being replaced by patterns in the media that represent servo information. The patterns are grooves and ridges formed in the servo wedges of the media of a disc. The grooves and ridges are formed during the manufacture of the disc, and occupy less area on the media than the bursts do. As a result, a greater area of the media may be used to store data or instructions.
Disc drives are being produced with increasing track densities and decreasing access times. A read/write head must fly over the media of a disc as closely as possible to read data and instructions from or write data and instructions to tracks that are closely spaced in the media. The patterns in the media abruptly change the fly height of a slider as it flies over one of the servo wedges, and therefore disturb the fly height of the read/write head in the slider. The disturbance increases the possibility of an error in reading from or writing to the media.
Several efforts have been made to improve the control the fly height of a read/write head. The load spring which forces the slider toward the media has been designed to influence fly height, and the shape of the slider has been altered in an attempt to improve its aerodynamics. However, none of the efforts have resulted in a suitable solution to the problems mentioned above. There remains a need for a system to control the fly height of a read/write head to allow it to read data from or write data to closely spaced tracks in a patterned media.
According to one embodiment of the present invention, a fly height of a read/write head in a disc drive is actively controlled as the read/write head is positioned over a rotating disc with a patterned media. The patterned media includes ridges and grooves in servo wedges in the patterned media. According to another embodiment of the present invention, an estimated fly height of the read/write head is compared with a desired fly height to generate a control signal. The control signal is used to apply a voltage difference across a piezoelectric layer joining the read/write head to a slider to move the read/write head relative to the slider and adjust the fly height of the read/write head. According to another embodiment of the present invention, a map of ridges and grooves in the patterned media is generated and the fly height of the read/write head is actively controlled based on information in the map.
Advantageously, the embodiments of the present invention provide for an active control of the fly height of a read/write head over patterned media to allow it to read data from or write data to closely spaced tracks. The active fly height control provides improved response to abrupt changes in the air flow boundary condition of an air bearing surface of a slider carrying the read/write head by moving the read/write head relative to the slider. The read/write head has a low mass relative to the slider and therefore a high resonant frequency which allows for a quick compensation of its fly height in response to the patterned media. The fly capability of the slider is maintained by allowing its fly height to change while moving the read/write head relative to the slider to maintain its capability to generate a useful read/write signal. The embodiments of the present invention help to preserve the sensitivity of the read/write head, and substantially reduces unwanted contact between the read/write head and the patterned media.